The invention relates to electrical circuit assemblies, and more particularly to gate return and current sensing connection and lead structure for such circuitry.
Electrical circuit assemblies typically include an electrically insulating thermally conductive substrate, such as ceramic, having a plurality of electrically conductive lead frames, such as copper, mounted on the substrate, and various semiconductor elements mounted on the lead frames and enclosed by an electrically insulating housing. Examples of such circuit assemblies are shown in U.S. Pat. Nos. 3,958,075, 4,156,148, 4,196,411, 4,215,235, 4,218,724, 4,250,481, 4,266,140, 4,394,530, 4,449,165, 4,449,292, 4,488,202, 4,498,120, 4,546,410, 4,546,411, 4,554,613, 4,574,162, 4,577,387, 4,630,174, 4,700,273. In other circuit assemblies, such as type TO-218, the ceramic substrate is deleted from the bottom of the housing, and the lead frame pads are left exposed when supplied to the user. The user mounts the package to a ceramic or other substrate according to his particular application.
The present invention involves the physical interconnection structure in such circuit assemblies for gate return referencing, and also for current sensing applications. The latter enables current mode control of switched mode power supplies, which has now been recognized as a better approach than voltage controlled methods, as noted in "Current Sensing HexSense Power MOSFETs Simplify SMPS Designs and Lower Losses", Sean Young, PCIM (Power Conversion and Intelligent Motion Magazine), July, 1987, pages 76-83. As noted in the Young article, current mode control has advantages of improved stability, automatic feed forward compensation for input voltage variations, pulse by pulse current limiting, and ease of paralleling supplies. The current mode control approach has become popular due to a variety of integrated circuits available to handle the control functions. A disadvantage to the current mode control has been the lack of an efficient means of monitoring instantaneous values of currents in the switching devices.
As noted in the Young article, in the past the current sensing function was usually done using either a sensing resistor or a current transformer in series with the switching device. The disadvantage of the series resistor is that it must always handle energy-wasting high heat dissipation, and its ohmic value must be chosen as a compromise between keeping such dissipation low, while at the same time generating a large enough signal. The disadvantage of the current transformer approach is that it is a magnetic component prone to saturation.
The Young et al article notes a current sensing power MOSFET component, the HexSense, which provides current sensing with negligible electrical losses. Such components are identical to standard power MOSFETs except that current from a few MOSFET cells are diverted to a separate source pin providing a known ratio of total current. Another pin, known as the Kelvin source, is connected to a point on the main source metallization. This Kelvin connection is the return point for the sense current. The voltage drop across the Kelvin pin is negligible and is unaffected by the magnitude of the main source current. This arrangement avoids errors in current sensing accuracy that would result if a voltage drop existed between the return point and the source metallization.
The present invention provides current sensing without an auxiliary sensing resistor and without a transformer. The invention utilizes standard semiconductor chips, such as standard MOSFETs, without the need for specialized MOSFETs such as the HexSense in the Young article.
The present invention provides lead frame terminal and semiconductor chip pad interconnection bonding structure enabling direct current sensing.
The invention also provides lead frame terminal and semiconductor chip pad interconnection bonding structure enabling direct gate return referencing. The invention minimizes the inductance otherwise present in previous designs using part of the source terminal for the gate reference return. This direct gate return referencing is accomplished without plural laterally adjacent bonds on the source pad with otherwise restrict the size of terminal wires which can fit within a given lateral size of such pad.
The invention also enables direct junction temperature sensing.
In the preferred embodiment, gate reference return inductance is minimized by providing a direct connection at the source pad for the gate reference terminal. Such inductance is also minimized by substantially eliminating the portion of the main source terminal through which both source current and gate return current flow.
Further in the preferred embodiment, a current sense terminal is connected to the source terminal at a point spaced from the source pad of the chip to define a given length of the source terminal therebetween. The gate return terminal directly connected to the source pad additionally serves as a current sense terminal such that the current flow through the source pad and the source terminal may be sensed according to the IR drop across the noted given length of the source terminal, without the need of an auxiliary resistor.